In an organic EL (electro-luminescence) panel, an organic EL element, arranged for each pixel, emits light to display a picture on a panel. The method for displaying a pixel may be classified into (1) a method by voltage driving and (2) a method by current driving. With the method by voltage driving, the voltage is applied to a transistor of each pixel. This transistor transforms the voltage into the current which current then flows through an organic EL element to emit light. With the method by current driving, the current is delivered to the transistor of each pixel. A current proportionate to the so delivered current then flows through the organic EL element of each pixel to emit light. In the case of the voltage driving, the current flowing through the organic EL element undergoes variations due to manufacture tolerances of the transistor of each pixel when the latter transforms the voltage into the current. Hence, the current driving is higher in accuracy than the voltage driving. However, with the current driving, it is necessary to deliver the current to a panel load to each pixel in addition to the current to be delivered to the transistor of each pixel. Hence, the smaller the current value, the longer is the time involved in driving.
The technique for enhancing the driving performance is disclosed in Patent Document 1. FIG. 13 depicts a block diagram of a current driving circuit disclosed in Patent Document 1. Referring to FIG. 13, the current driving circuit includes a current source 102 that drives a capacitive load 101 and an over-drive circuit 103, which is provided apart from the current source 102 to operate as a startup circuit for driving the capacitive load 101. The over-drive circuit 103 is controlled by an output of a band-gap voltage driving circuit 104 that detects the voltage of a load driving terminal 100. With a threshold value of, for example, 2.6V for a detection voltage of the load driving terminal 100, the current is supplied from both the current source 102 and the over-drive circuit 103 if the load terminal voltage is less than 2.6V. If the load terminal voltage is not less than 2.6V, the current from the over-drive circuit 103 is cut off such that only the current from the current source 102 is delivered. The startup response characteristic may thus be improved by the driving of only the current source 102 without changing the current value of the driving current (current value of the current source 102).
Meanwhile, as a related technique, a current limiting circuit, operating in stability against changes in the power supply voltage and in the operating temperature, is disclosed in Patent Document 2.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2000-278107A
[Patent Document 2]
JP Patent Kokai Publication No. JP-A-02-128205